<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>id选择器</title>
    <style type="text/css">
        #firstId{
            color: red;
            font-size: 20px;
            border-bottom: mediumblue 10px double;
            width: 95px;
            background: lawngreen;
            margin: 0;
            padding: 0;
        }
    </style>
</head>
<body>
<h3 id="firstId">id选择器</h3>
<!--同一页面内的id值必须唯一-->

</body>
</html>